Colour picture tube including shadow mask having self-compensation function for thermal stress

ABSTRACT

The peripheral portion of a shadow mask which is spot welded to a supporting frame is provided with a plurality of openings so as to form a cushion zone for absorbing and minimizing the stress created therein by heat.

United States Patent [1 1 Ishizuka et al.

[111 3,862,448 5] Jan. 21, 1975 COLOUR PICTURE TUBE INCLUDING SHADOWMASK HAVING SELF-COMPENSATION FUNCTION FOR THERMAL STRESS Inventors:Tomio Ishizuka; Hiromi Kanai;

Takao Kawamura; Kenichi Fukuzawa, all of Mobara, Japan Assignee:Hitachi, Ltd., Tokyo, Japan Filed: Aug. 31, 1972 Appl. No.2 285,322

Foreign Application Priority Data Sept. 3, 1971 Japan 46-79365 Nov. 1,1971 Japan 46-100852 US. Cl. 313/402 Int. Cl. H01j 29/06 Field of Search313/85 S, 92 B, 402

[56] References Cited UNITED STATES PATENTS 2,733,366 l/1956 Grimm etal. mm B x 2,795,718 6/1957 Van Hekken et al. 313/85 S 3,524,974 8/1970Kautz .f3l3/85 S 3,585,431 6/1971 Long 313/85 S 3,639,799 2/1972Kirchner 313/85 S 3,660,708 5/1972 Nakamura et al.. 313/85 S 3,809,9455/1974 Roeder 3l3/402 Primary ExaminerSiegfried H. Grimm Attorney,Agent, or FirmDike, Bronstein, Roberts, Cushman & Pfund [57] ABSTRACTThe peripheral portion of a shadow mask which is spot welded to asupporting frame is provided with a plurality of openings so as to forma cushion zone for absorbing and minimizing the stress created thereinby heat,

2 Claims, 3 Drawing Figures COLOUR PICTURE TUBE INCLUDING SHADOW MASKHAVING SELF-COMPENSATION FUNCTION FOR- THERMAL STRESS BACKGROUND OF THEINVENTION This invention relates to a colour picture tube and moreparticularly to the improvements of the colour selection electrode of acolour picture tube.

In a colour picture tube wherein a shadow mask is used as the colourselection electrode, electron beams emitted from an electron gunassembly are caused to impinge upon a fluorescent screen comprising aplurality of trios of phosphor dots for three colours through the shadowmask assembly mounted in front of the fluorescent screen, therebyreproducing a picture image thereon.

Generally, the shadow mask assembly comprises a shadow mask with aplurality of small perforations and a supporting frame welded to theperiphery of the shadow mask and the shadow mask assembly is supportedin an evacuated envelope of the colour picture tube through a pluralityof leaf springs. Usually the supporting frame is welded to the peripheryof the shadow mask with a suitable spacing therebetween. However, sincethe shadow mask is made of an extremely thin metal sheet having athickness of about 0.2mm which is provided with a plurality of smallperforations, maldistribution of a small stress will cause deformationof the shadow mask.

Even when the stress is prevented at the time of welding since the frameis welded to the shadow mask at spaced apart pointsstress will becreated in the shadow mask due to various heat treatments performed atvarious fabrication steps of the colour picture tube and the heatgenerated by the collision of the electron beams during the use and testof the completed tube. The central portion of the shadow mask isprovided with a plurality of small perforations whereas the peripheralportion is not perforated so that these two portions have differentmechanical strength with the result that the heat expansion of theshadow mask caused by the collision of the electron beams will causeabnormal deformation of the interface between the central and theperipheral portions. Such a deformation is enhanced by the facts thatthe peripheral portion is secured to the frame having a large heatcapacity and that the electron beams are shielded by the mask frame andan electron shield secured thereto.

The same problem also occurs in the slit type colour selectionelectrode.

SUMMARY OF THE INVENTION colour picture tube including an improvedcolour selection electrode which will not be deformed by the heatgenerated during the fabrication and use of the colour picture tube.

A further object of this invention is to provide a colour picture tubeincluding an improved colour selection electrode which is constructed toprevent abnormal local deformation caused by thermal expansion of thecolour selectionelectrode.

According to this invention there is provided a colour picture tubecomprising a colour selection electrode assembly including an operatingportion adapted to transmit an electron beam, a peripheral portionsurrounding the operating portion, and a supporting frame welded to theperipheral portion, characterized in that a cushion zone is provided forthe peripheral portion for absorbing and alleviating the stress createdtherein due to heat.

The cushion zone is formed by providing a plurality of smallperforations, blind openings or grooves in the peripheral portion ordecreasing the thickness thereof.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a perspective view of a shadow mask assem bly embodying theinvention and FIGS. 2 and 3 show partial sectional views of modifiedshadow masks.

DESCRIPTlON OF THE PREFERRED EMBODIMENTS tions 21 for transmittingelectron beams, whereas the peripheral portion 23 surrounding thecentral portion is not perforated. The peripheral side 24 is providedwith cushion zones 26 formed with small perforations 25. While in theillustrated example small perforations are formed throughout the entiresurface of the peripheral side 24, it should be understood that suchperforations may be formed in areas around equally spaced apart spotwelds which interconnect the side 24 to a supporting frame 30.

Cushion zones 26 with small perforations absorb or alleviate the stressformed in the shadow mask 20 and I or frame 30. Consequently, the shadowmask 20 is prevented from being deformed by the thermal stress createdtherein by the heat generated by the high temperature treatment carriedout during the fabrication of the colour picture tube or generatedduring normal use thereof, thereby displaying a picture image ofexcellent quality without any degradation of the colour purity.

The result of experiment shows that in a shadow mask of thin metal sheethaving a thickness of about 0.15mm wherein the central portion thereofis provided with a plurality of small perforations at a pitch of about0.50, 0.60 or 0.70mm, a diameter of from 0.15 to 0.45mm of the smallperforations 25 in the cushion zones 26 and a density of from 10 to ofthe perforations 25 give satisfactory results.

It will be clear that the configuration and size of the smallperforations 25 in the cushion zones 26 may be decided properly indesign in accordance with the number of spot welds and the like. Inother words, the small perforations are adapted merely to form cushionzones 26 for stress acting between the operating portion 22 of theshadow mask 20 having perforations 21 and the spot-welded portions ofthe peripheral side 24. From the standpoint of manufacturing it isadvantageous to make equal the diameters of the perforations 21 in the Ioperating portion 22 and the perforations 25 in the side 24.

In the embodiment shown in FIG. 1 since a rigid peripheral portion 23 isinterposed between the central portion 22 and the peripheral side 24 ofthe shadow mask it is impossible to efficiently absorb or alleviate thestress caused by the expansion and contraction of the shadow mask and/orframe 30. This problem can be solved by a modified embodiment shown inFIG. 2 wherein the shadow mask is constructed to have sub stantiallyuniform mechanical strength throughout the entire surface thereof.

In FIG. 2, a cushion zone 26 is formed in both'peripheral portion 23 andthe peripheral side 24 of the shadow mask 20. The cushion zone isprovided with a plurality of small blind openings 28, preferably havingthe same diameter as the perforations 21 at the central portion 22.Provision of a plurality of small blind openings 28 decreases theeffective thickness of the peripheral portion 23 and the side 24, thusassuring substantially uniform mechanical strength over the entire areaof the shadow mask. With this modification, the shadow mask 20 expandsin the direction of the passage 40 of the electron beam as shown bydotted lines. For this reason, the shadow mask 20, especially thecentral portion thereof is freed from any abnormal deformation caused bythermal expansion thereby preventing the degradation of the colourpurity and colour shading of the colour picture tube. In FIG. 2, numeral42 indicates a panel of the envelope of the tube.

In another modification shown in FIG. 3, the thickness of thepheripheral portion 23 and the side of a shadow mask is decreased toform a cushion zone which has the same effect as the embodiment shown inFIG. 2. The dot and dash lines show the original thickness of thecushion zone.

Although in the embodiments shown in FIGS. 2 and 3, the cushion zone 26is formed by providing a plurality of small blind holes or by decreasingthe thickness of the cushion zone, grooves may be provided for this zoneprovided that the uniform mechanical strength of the shadow mask can beassured.

Although FIGS. 2 and 3 show one example where the outer side (so-calledpanel side) of the shadow mask is cut away, it should be understood thatthe invention is also applicable to formation of the cushion zones byeliminating the inner side of the shadow mask.

Although the invention has been described in terms of some preferredembodiments thereof it will be clear that many changes and modificationswill be obvious to one skilled in the art within the scope of theinvention as defined in the appended claims. Further, it should beunderstood that the invention is also applicable to other types of thecolour selection electrode. for example of the slit type or zig-zagtype.

What is claimed is:

1. In a color picture tube comprising a colour selection electrodeassembly including an integrally formed operating portion adapted totransmit an electron beam, a peripheral portion surrounding saidoperating portion and a side portion; and a supporting frame welded tosaid side portion, the improvement which comprises a cushion zone formedby weakening the mechanical strength continuously over the area ofthermal stress transmission in said peripheral portion and said sideportion through provision of a uniformly distributed plurality of smallblind openings over said area to obviate maldistribution of thermalstress in the mask.

2. In a color picture tube comprising a colour selection electrodeassembly including an integrally formed operating portion adapted totransmit an electron beam, a peripheral portion surrounding saidoperating portion and a side portion; and a supporting frame welded tosaid side portion. the improvement which comprises a cushion zone formedby weakening the mechanical strength continuously over the area ofthermal stress transmission in said peripheral portion and said sideportion through provision of a reduced thickness over said area toobviate maldistribution of thermal stress in the mask.

1. In a color picture tube comprising a colour selection electrodeassembly including an integrally formed operating portion adapted totransmit an electron beam, a peripheral portion surrounding saidoperating portion and a side portion; and a supporting frame welded tosaid side portion, the improvement which comprises a cushion zone formedby weakening the mechanical strength continuously over the area ofthermal stress transmission in said peripheral portion and said sideportion through provision of a uniformly distributed plurality of smallblind openings over said area to obviate maldistribution of thermalstress in the mask.
 2. In a color picture tube comprising a colourselection electrode assembly including an integrally formed operatingportion adapted to transmit an electron beam, a peripheral portionsurrounding said operating portion and a side portion; and a supportingframe welded to said side portion, the improvement which comprises acushion zone formed by weakening the mechanical strength continuouslyover the area of thermal stress transmission in said peripheral portionand said side portion through provision of a reduced thickness over saidarea to obviate maldistribution of thermal stress in the mask.